1. Field of the Invention
The present invention is directed to patterned polymers and a method of forming patterned polymer substrates using initiated chemical deposition operated under supersaturated conditions.
2. Description of the Related Technology
Poly(vinylpyrrolidone) or PVP is a water soluble polymer made by the polymerization of N-vinylpyrrolidone (VP). With a molecular weight (Mw) ranging from 2500 to 1 million, PVP is mainly produced using free radical addition polymerization. Vinylpyrrolidone can be polymerized using organic peroxide initiators, such as di-tert-butyl-peroxide (TBPO) or dicumyl peroxide. In the liquid phase, polymerization is initiated by solvent radicals that are produced through hydrogen abstraction from alkoxy radicals formed by initiator decomposition. Chain propagation then occurs normally and termination occurs through hydrogen abstraction to form an alkyl end group and a solvent radical. The solvent radical reinitiates the polymerization process to form a new chain.
PVP has several important properties that provide versatility for use in a variety of industrial applications. Commercially available PVP polymers are highly polar, amphiphilic and compatible with a variety of resins and electrolytes. PVP also has adhesive and cohesive properties, is physiologically inert and is also cross-linkable and can be used to produce hard, glossy, oxygen permeable films which can adhere to a variety of substances. PVP is also soluble in water as well as in various organic solvents due to its combination of both hydrophobic and hydrophilic functional groups, which makes interactions with a wide variety of solvents possible. Commercially produced PVP is soluble in most polar solvents and insoluble in less polar solvents. However, PVP produced without water can also be dissolved in solvents such as dioxane, acetone and toluene.
PVP can be used in fields such as pharmaceuticals, cosmetics, foods, adhesives, textiles and the renewable energy industry. In pharmaceuticals PVP is used as a binder, a coating and disintegrant for tablets, and a solubilizer for drug suspensions. PVP is a stabilizer in the beverage industry. For uses in the pharmaceutical and beverage industry, the interaction between PVP and low molecular weight compounds is crucial. PVP has a tendency to form complexes with different compounds (inorganic anions, amino acids, phenols and proteins) and polymers of both low and high molecular weights. This property can sometimes be beneficial because PVP can solubilize insoluble substances. However, for the pharmaceutical industry strong complexes can reduce the bioavailability of the active substance. Therefore, interactions between drug compounds and PVP have to be investigated thoroughly before use.
For the cosmetic field, PVP is used as a film former in hair sprays, lotions and conditioning shampoos. It is also used in glue sticks as an adhesive to improve strength and toughness. In the textile industry, PVP is utilized as a dye-affinitive stripping and leveling agent. It also has uses as a polymer thin film in supercapacitors and as a polymer gel electrolyte in dye sensitized solar cells. PVP is a versatile polymer that has the potential for many new applications.
Initiated chemical vapor deposition (iCVD) is a technique used to deposit polymer thin films under vacuum. Monomer and initiator vapors flow into the reactor where a heated filament, located inside the chamber, activates the initiator. Next, both the activated initiator and monomer adsorb onto the surface of the substrate. The activated initiator then begins the polymerization process by linking monomer units together to form the polymer chain. A schematic representation of the iCVD process is shown in FIG. 1. The advantages of iCVD include its lower temperature of operation compared to other hot wire CVD techniques as well as the relative chemical purity and physical uniformity of polymer films produced on surfaces by iCVD.
iCVD is a proven technique for numerous applications, including hydrogels, solar cells, sensors, and various thin film coatings. Compared to other film deposition techniques, like spin coating, iCVD does not use liquid solvents during processing. This is particularly attractive as the use of liquid solvents during processing can often result in a decrease in cell performance from solvent residue and solvent incompatibilities with existing cell components. Also, the iCVD process occurs in a single step, enabling simultaneous polymerization and coating deposition, and provides the necessary physical and chemical control needed for device fabrication.
The deposition rate of polymers using iCVD may be limited by the monomer availability at the surface. Therefore, a lower substrate temperature results in faster deposition as this increases monomer adsorption. The amount of monomer adsorption can be expressed in terms of the fractional saturation at the surface, which is the ratio of the monomer's partial pressure to its saturated vapor pressure (Pm/Psat) at the temperature of the surface. At low values of Pm/Psat, monomer adsorption corresponds to less than one layer of film and the surface concentration increases linearly with Pm/Psat. At higher values of Pm/Psat, there is multilayer adsorption of monomer. At a Pm/Psat>1, or supersaturated conditions, the monomer is known to condense on the surface. Typically, under supersaturated conditions, films are not smooth and uniform due to the formation of condensed, coalesced monomer droplets. Therefore, to avoid condensation and in order to produce uniform, homogeneous films, iCVD is usually run at intermediate Pm/Psat conditions of between 0.4-0.8 to ensure a reasonable deposition rate. See, Lau, K. K. S, Gleason, “Initiated chemical vapor depositions (iCVD) of Poly(alkyl acrylates): a kinetic model,” Macromolecules 39 (2006) 3688-3694.
There are only a few studies that have been done to examine the deposition behavior and morphology of films produced under supersaturated conditions (Pm/Psat>1). See, Tao, R., Anthamatten, M., “Condensation and polymerization of supersaturated monomer vaport,” Langmuir 28 (2012) 16580-16587. In the deposition of poly(glycidyl methacrylate) (PGMA) films under supersaturated conditions, surface undulations were observed. As the level of supersaturation increased (greater Pm/Psat values) the period and amplitude of the undulations increased as well. This may be due to the monomer condensing on the substrate followed by the nucleation of droplets and film growth.
One of the most important properties of monomer condensation is the wettability of the solid surface with the condensing liquid. Wettability can be quantified using the contact angle of the liquid on the solid. It has been shown that depending on the wetting properties of the solid surface, liquid condensation can occur either in the form of a droplet or a thin film. If complete wetting is achieved by the liquid, condensation will occur as a thin film, otherwise droplets and islands will form. See, Zhao, H., Beysens, D., “From droplet growth to film growth on a heterogeneous surface: condensation associated with a wettability gradient.” Langmuir 11 (1995) 627-634.
With the accelerated development of micro- and nanoscale systems, there is a concurrent need for more effective fabrication approaches. Particularly for soft organic polymer in micro- and nanoelectromechanical systems (MEMS, NEMS), hybrid organic-inorganic systems, organic semiconductor electronics, organic photovoltaics, and optoelectronic devices, there is a need for suitable polymer patterning techniques to create well-designed micro- and nanostructured surfaces with well-maintained polymer properties.
Diverse strategies in polymer patterning have been reported and reviewed. In general, the current strategies can be divided into five approaches: (1) molding, (2) photolithography, (3) stamping, (4) self-organization, and (5) direct writing. The molding approach employs elastomeric molds to stamp or print a liquid precursor solution on defined locations. It is capable of patterning polymers on the nanoscale but requires time for the precursor solution to diffuse into the microchannels of the mold and dry. See, Guo, L. J., “Nanoimprint Lithography: Methods and Material Requirements.” Advanced Materials (2007), 19, 495-513. In the photolithography approach, polymer patterns can be sharp and made with a high throughput but it requires solvent-based photoresists and possibly causes polymer material damage from radicals generated during UV irradiation are undesirable. See, Jensen, J.; Dyer, A. L.; Shen, D. E.; Krebs, F. C.; Reynolds, J. R., “Direct Photopatterning of Electrochromic Polymers.” Advanced Functional Materials (2013), 23, 3728-3737. The stamping approach generates a chemically contrasted surface by “ink” solutions. Therefore, solvent evaporation is needed between each stamping step thereby slowing down throughput. In self-organization, polymer patterns are highly periodic and defect-free but they are hard to design since the patterns are strongly dependent on polymer physics and chemistry. See, Nurmawati, M. H.; Renu, R.; Ajikumar, P. K.; Sindhu, S.; Cheong, F. C.; Sow, C. H.; Valiyaveettil, S., “Amphiphilic Poly(p-phenylene)s for Self-Organized Porous Blue-Light-Emitting Thin Films.” Advanced Functional Materials (2006), 16, 2340-2345. Besides, designed polymers need to be dissolved for film spreading, which requires solvent removal. The direct writing approach, such as dip-pen nanolithography can write nanoscale patterns directly on a surface by using a nanoscale probe tip with the desired material solution. See, Piner, R. D.; Zhu, J.; Xu, F.; Hong, S.; Mirkin, C. A., “Dip-Pen Nanolithography.” Science(1999), 283, 661. However, throughput is relatively slow. For all of the currently available approaches, polymer patterns are made using solvent-based procedures requiring solvent removal.
Currently, photolithography is the most common method of patterning thin films. Photolithography is an optical technique to transfer patterns from a photomask to a polymer layer coated on a substrate. The desired pattern is first applied on a photoresist layer. The pattern is then developed into a mask that creates a patterned film by selectively etching the underlying layer. The whole process involves surface preparation, coating, pre-bake, alignment, exposure, development, post-bake, processing using the photoresist as a masking film, stripping and finally post process cleaning. Photolithography needs to be done in a clean room and the process is long and tedious. Spin coating is the most common coating method used. However, problems such as drying during spinning increases the viscosity of the resist edges. The edges can be up to 20-30 times the average thickness of the resist. There is also a high possibility of streaks, which are caused by particles with a diameter greater than the thickness of the photoresist layer. See, Fransilla, S., Lithography, in “Introduction to Microfabrication,” W. Sussex, UK: John Wiley & Sons Ltd. 2004, ch. 9, sec. 9.1-9.4, pp. 99-103. All of these defects reduce the quality of the pattern.
Surface patterning has many biological application such as tissue scaffolding, stem cell research, and to control cell behavior. What is needed is an easier, faster alternative to photolithography for patterning polymer thin films.